Axial piston machines are known in a wide variety of configurations for use as motors and pumps and have been described generally in my application Ser. No. 558,273 filed 14 Mar. 1975 and earlier patents issued to me alone and jointly with others. Such machines have the advantage that, generally speaking, the stroke and operating capacity can be varied by the displacement of a control element.
A typical axial piston machine (pump or motor) which may be used in a variable speed hydrostatic transmission or in any other hydraulic force transmission system, comprises a housing formed with a valve plate or other surface having a pair of kidney-shaped apertures connected respectively to an inlet and an outlet for the hydraulic fluid media. A cylinder drum is rotatable against the valve plate or surface and is formed with a plurality of angularly spaced cylinder bores which surround the rotation axis of the drum and are generally parallel thereto, receiving respective pistons. The pistons project from the cylinder drum and, at their exposed extremities act against a control surface which may be tilted about an axis perpendicular to the axis of the drum. In a tilted position of this control surface (e.g. a control plate also known as a swash plate) certain pistons tend to extend out of the cylinder drum while other pistons tend to recede into the latter as the drum is rotated so that the stroke of each piston represents the excursion between its innermost and outermost positions. When the drum is driven by a shaft journaled in the housing, the pistons are alternately forced into and out of their respective bores to expel fluid through a discharge port and induce fluid through the oppositely functioning or inlet port so that the machine operates as a pump. When, however, fluid is forced into an inlet port and urges pistons outwardly from retracted positions, a shaft connected to the drum directly or indirectly is compelled to rotate and the machine operates as an axial-piston motor.
When the control surface lies in a plane perpendicular to the axis of rotation of the drum, the pistons undergo no excursion with drum rotation and the machine is said to be operated in a neutral mode. In this mode, when the machine is a pump, there is effectively no displacement and, when the machine is a motor, the output shaft is at standstill.
In general, the displacement of the control surface, i.e. a control plate or swash plate, is effected by a hydraulic servomechanism having a cylinder for a piston linearly shiftable in the cylinder and coupled to the pivotal plate by a complex linkage designed to convert the linear movement of the actuating piston to a tilting movement of the control plate. Generally this linkage consists of a plurality of elements or bodies, i.g. links or levers, of relatively high cost. Furthermore, such multielement linkages are not able to provide a play-free positioning of the control surface without mechanically stressing the elements to an undesirable extent and producing a binding or the like. For the most part, such systems are prone to wear, require relatively large stroke for the servopistons and therefore a large length with numerous difficulties with respect to constructing the servomechanism in the housing for the axial piston machines. Manufacturing costs are high, the weight of the unit is increased and the capacity per unit size is reduced by the need for a complex large-stroke servomechanism.